Under an agreement reached in March, the pair – once they sign the documents – would continue their interim arrangement of joint legal and physical custody of their two sons.

"Britney's attorneys are anxious to see Kevin sign off on a divorce," the source says. "But Kevin and his lawyer Mark Vincent Kaplan are dragging their heels out of concern over reports of Britney's post-rehab partying."

Federline's camp, the source adds, "wants to make sure that the divorce document makes it simple enough for Kevin to downsize Britney's access to her kids the next time her behavior troubles him."

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